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N. W. PIRIE 



of metabolic expertise. Not completely however; we still use iodine in the thyroid 

 and normal hfe depends on this otherwise exceptional synthesis. There is no 

 reason to think that this use of iodine is new; it is found in present-day amphibia 

 and fish and may well be ancient, I suggest that it is a reUc of an initially more 

 cathoUc approach to metabolism and that evolution has not eliminated it because 

 there are few regions where the amount of iodine in rock is so small as to make 

 this dependence a disadvantage. The only logical alternative is to look on the 

 thyroid as the remains of a commensal sponge that lodged in the gill arches of 

 a primitive vertebrate giving rise ultimately to symbiosis as intimate and essential 

 as that in the lichens. 



Fig. I. At each level of time the width of the cone represents the number of 

 ways in which living or Ufelike systems worked. The size of the atomic symbol is 

 an indication of the contribution that element may have made to the process at 



the time. 



The evidence that much of evolution is accompanied by the selective loss 

 rather than gain in capacity was marshalled by Lwoff [13]. This leads to the 

 conclusion that present-day organisms are descended from one or a few super- 

 organisms with more extensive capacities than those now found. It is necessary 

 to conclude that these super-organisms were far removed from eobionts and 

 were perhaps built up by symbiosis among several of them. The same type of 

 selection that Lwoff has called attention to probably then operated so that the 

 final organism would be derived as much by discarding or minimizing capacities 

 as by acquiring new ones. The metabolic processes used by present-day organisms 

 were probably present in some of the early forms of life but there is no reason 



